Boil the potatoes about 15 minutes. They should be cooked but still firm. Thaw and blanch the spinach. When the potatoes are cooled, dice them in about 3/8 inch cubes. Squeeze out the excess water from the spinach and mix it with the potatoes. Dice the red pepper or pimento and add it in together with the pine nuts. Chop the garlic and cilantro leaves. Put them in a mortar and start pounding into a paste. Add the chopped black walnuts and the salt and continue mashing. Mix in the lemon juice and cumin, then slowly add the olive oil while working the mixture into a thick ``mayonaise''. Fold this into the potatoes and spinach. Then crumble the fetta over the salad and fold it in gently.